One type of modern nonvolatile memory is the EPROM or EEPROM device that uses floating gate structures. These floating gate memory structures may be integrated into a floating gate array which facilitates interface between the memory cells, control circuitry and high-voltage power sources. The memory cells use channel hot electrons for programming from the drain side and use Fowler-Nordheim tunneling for erasure from the source side. Due to the high voltages frequently used to program and erase the cells, high-voltage peripheral transistors may be implemented to provide an interface between a high-voltage source and the memory cells of the floating gate array. The control logic circuitry of the floating gate array typically functions with a lower operating voltage. Low-voltage peripheral transistors may be implemented to provide the logic circuitry for the array.
It is often desirable to fabricate peripheral transistors and the floating gate memory cells on a common semiconductor substrate. To ensure that each cell operates independently, regions of isolation dielectric may be formed between cells to electrically isolate the gates from one another. Typically, the isolation dielectrics for the memory cells are formed at the same time and are of the same construction as the isolation dielectrics for the periphery transistors. In addition, to ensure appropriate coupling coefficients, the distal ends of floating gates in the memory cell area are made to overlap the isolation dielectric. The proper voltage applied to the control gate is proportional to the coupling coefficient of the device. Providing overlap of the ends of the floating gates over the isolation dielectric increases the coupling coefficient, allowing for a lower control gate voltage.
This approach has several disadvantages. One problem with this approach is that it is difficult to instill unique characteristics into the separate isolation dielectrics because they are formed contemporaneously. For example, memory cells frequently use high voltages for programming and erasing data. Periphery transistors interfacing high voltage sources typically implement thicker gate oxides and larger isolation dielectric regions than those typically associated with memory cell gate oxides. Additionally, as memory cell sizes are reduced, their corresponding gate oxides become thinner and isolation dielectric regions become smaller. An approach that contemporaneously forms isolation dielectrics for memory cells and peripheral transistors cannot satisfy theses diverging specifications.
Another problem with this approach is that as the memory cells are scaled, they become intolerant to low levels of leakage current. The read current of each memory cell is directly proportional to the area of substrate supporting the floating gate, and inversely proportional to the thickness of the oxide separating the substrate and the floating gate. To facilitate integration with other scaled system elements, the width of the floating gates is frequently decreased. Maintaining a desired overlap of the floating gate over the isolation dielectric, however, requires a corresponding decrease in the substrate area supporting the gate. Decreasing this area width without reducing the gate oxide thickness results in degradation of the cell's read current. To maintain a desired read current, therefore, the thickness of the gate oxide must also decrease. Unfortunately, it is often impractical to decrease the gate oxide thickness because this leads to high levels of stress-induced leakage current. Therefore, it is difficult to scale these devices without either degrading the cell's read current or making them susceptible to stress-induced leakage currents.
Another problem with this approach is that the overlapping regions result in complex device topography, making it difficult to etch all of the polysilicon from the areas between floating gates. Failure to remove all of the polysilicon between floating gates may cause adjacent floating gates to short out.
Still another problem with this approach is that when used in conjunction with a configuration comprising trenches and moats within the substrate, the gate oxide layer tends to thin around the trench corners, causing data retention and threshold voltage nonuniformities.